Metal-air battery cells have wide applicability to portable electronic equipment, such as lap-top computers, video cameras, and other types of hand-held devices. Metal-air battery cells provide relatively high power output with relatively low weight as compared to other types of electrochemical cells. These power and weight advantages are due in part to the fact that metal-air cells utilize oxygen from the ambient air as a reactant in the electrochemical process rather than a heavier material such as a metal or metallic composition.
Generally described, a metal-air cell, such as a zinc-air cell, uses one or more air permeable cathodes separated from a metallic anode by an aqueous electrolyte. During operation, oxygen from the ambient air is converted at the cathode to produce hydroxide ions while zinc is oxidized at the anode to react with the hydroxide ions. Water and electrons are released in this reaction to provide electrical energy. On recharge, voltage is applied between the anode and the cathode of the cell and the electrochemical reaction is reversed. Oxygen is discharged back to the atmosphere through the air permeable cathode and hydrogen is vented out of the cell.
High power output from metal-air cells has been accomplished through the use of a dual cathode or a dual air electrode cells. An example of such a dual air electrode cell is found in commonly-owned U.S. Pat. No. 5,569,551 which describes a metal anode, multiple layers of separator materials with liquid electrolyte substantially trapped therein, an upper and lower cathode, an upper and lower mask wall, side walls, and side vents. A further development is shown in commonly-owned U.S. Pat. No. 5,639,568 entitled "Split Anode for a Dual Air Electrode Cell." This patent describes a metal anode for a dual air electrode cell that has upper and lower metal anode layers and either a foil current collector therebetween or a two layer current collector with means for inhibiting zinc movement positioned between the two current collector layers. This design is effective in eliminating or reducing "slumping", i.e., the migration of zinc from one layer to another. Slumping may contribute to capacity loss, operating voltage loss, and may cause an imbalance in current distribution between the cathodes.
A drawback with the current design of metal-air cells is that the cells tend to lose power capability during storage, particularly start-up power. This problem may be due to the manner in which the zinc anode operates while being cycled. During discharge, the zinc discharges from the outside interface with the cathode back towards the current collector. As the anode discharges, zinc is converted to zinc oxide such that the zinc oxide forms from the cathode interface towards the current collector. On recharge, the bulk of the zinc electrode also charges from the cathode interface on the outside back towards the current collector, i.e., the bulk of the zinc forms from the cathode interface towards the current collector.
This pattern may create the situation in which the anode, if not fully charged, has a layer of zinc oxide between the charged zinc at the interface with the cathode and the current collector. In other words, the current collector is not in sufficient contact with the metal zinc to provide minimum power. The zinc oxide layer apparently somehow passivates during storage. This passivation may be due to a change in the morphology of the zinc oxide or by the zinc oxide forming over any metal zinc bridges to the current collector. Passivation leads to a higher resistance in the anode and creates a power loss for the cell as a whole. This problem is particularly of concern at start-up. Although metal-air cells tend to have a power dip for the first several minutes after start-up and before the cell achieves its rated voltage, the power loss problem becomes more acute after the cell has been in storage for a given length of time.
There is a need, therefore, for a metal-air cell that avoids passivation and, more particularly, limits the loss of power at start-up. Such a design would limit the loss of power at start-up so as to provide a more reliable cell. These goals must be accomplished in a cell that remains light-weight and relatively inexpensive for wide spread consumer use in any type of portable electronic device.